An industrial robot system comprises a manipulator and control equipment, whereby the manipulator with the assistance of the control equipment carries out arbitrary operations within a working range. Usually, such a manipulator comprises a plurality of links which support a wrist and a tool flange, on which a tool is arranged. In the majority of robot applications, a traditional six-axis manipulator is used, which exhibits sufficient movability to carry out a wide range of operations with mostly very high accuracy. The control equipment includes a path generator for generating a robot path. The path generator receives instructions from a control program and on basis thereof determines how the manipulator should move in order to be able to execute the movement instruction. For example, the path generator calculates motor torque references and position and velocity of the axes of the robot. The computed motor references are transmitted to one or more drive modules to drive the manipulator in accordance with the movement instructions.
Depending on more intense utilization of robots, shorter change-over time and reduced scrap-rate in automated tasks, maintenance of industrial robots should be enabled with minimum breakdown times.
Service maintenance is traditionally operated by a breakdown maintenance strategy, i.e. unscheduled maintenance where a plant item is run to failure, or by a time based maintenance strategy, i.e. scheduled maintenance stop intervals, where a plant item is maintained at periodic intervals. While the former has long down-time and hence can present risks of loss in production and induce high repair costs, the latter may lead to unnecessary maintenance or even induce failures.
Condition based maintenance has been increasingly used for reliable, cost-effective monitoring of selected working characteristics of plant items. By moving from reactive to predictive maintenance, an organization can expect to reduce inventory costs out of maintenance and repair operations with no negative impact on productivity or availability of the facility. A prediction of the reliability of a single piece of equipment on the plant floor can be the difference between a few minutes of preventive maintenance and hours of downtime.
An automatic and simple to use method for health condition indication of a robot manipulator is hence a key requisite for a minimum breakdown maintenance strategy.
There are today, fairly manual methods indicating “health condition” of a robot manipulator. This task is usually addressed to the skill of a service engineer to “hear” and “interpret” different “signals”.
An actual movement of a manipulator of an industrial robot in production is not for certain the same from cycle to cycle of repeated cycles. The robot system interacts with its environment, e.g. via I/O ports. The manipulator can be instructed to wait for other machines to finish their work or wait for a specific signal. This means that not all parts of data sampled from signals related to manipulator movement cycles collected during production are directly comparable from one cycle to another cycle of the repeated cycles.
A problem to be solved by the present invention is to how to synchronize signals logged at two different work cycles of the robot in order to be able to perform condition change analyses in the cases where the robot movements are not identical during both cycles, so the collected data is not directly comparable for the whole cycle.
Repeatability analyses as a tool for monitoring the robot performance requires possibility of comparing motion data sampled at different occasions. This in turn requires a technique with help of which synchronization of such signals is possible. Traditionally, such a comparison is performed utilizing test cycles, especially designed for this purpose.
U.S. Pat. No. 4,150,326 discloses a method to compare stored positional data representations of the manipulator during an observed operational work cycle with actual positional data representations during subsequent work cycles. The positions are measured at predetermined time intervals during execution of instructions in an instruction list and it is detected when a position during a time interval differs from previous measurement at this time interval. Such a method could be used for synchronization of signals logged at two different work cycles of the robot. However, a disadvantage with using this method for synchronization is that it requires total knowledge of the instruction list, hence the method must be performed in the control system associated with the robot, or the instruction list must be available to a stand alone apparatus. Further, a change in the instruction list spoils the synchronization. In a robot system a change in one instruction will not only affect the movement when execution this instruction, but also the instructions next to it. This method is not capable of handling these situations. Further, any non deterministic instructions, e.g. instructions where the robot waits for an external condition, will spoils the synchronization.
US 2005/0278148 discloses a method for appraising wear of axis of a robot arm. The method does not include steps of synchronizing measurements from different occasions and can not in a direct matter compare or pick out differences between data collected at two different occasions. Only using measured signals, e.g. the torque, not reference signals, and using trying to synchronize data from different occasions has several disadvantages. One disadvantage with this method is that the synchronization does not work in case the robot has to stand still and wait for something in one cycle and not in the other cycle, i.e. in cases where the robot movements are not identical during both cycles. A further disadvantage with this method is that it uses measured signals. The measured signals include noise, which means that it is not possible to synchronize each sample of the measured signal. This leads to a bad accuracy of the synchronization.
U.S. Pat. No. 5,822,212 and U.S. Pat. No. 7,069,185 discloses a condition change analyses, which is performed by comparing logged work cycles using a clock signal as reference. However, the clock signal does not contain any information on the robot motion and cannot be used for synchronization of signals logged at two different work cycles of the robot in cases where the robot movements are not identical during both cycles.